I've never been a fan of the /roll as I've seen it used too often to basically negate the creativity of a more skilled RPer. For example:
Player A [dwarf] is being robbed/attacked by Player B [kran].
Player B swings kra's club down at Player A's head.
Player B /roll 100 - 5, Player A /roll 100 - 76, club misses.
Player A ducks under the club, and while A is not particularly agile, A has spent decades climbing around mines in search of the most productive veins of ore. A tries to scrabble up B's arm, trying to climb up onto kra's shoulders and out of the reach of the club.
Player A /roll 100 - 65, Player B /roll 100 - 87, doesn't climb.
Player B moves out of the way.
So on and so forth. I personally like it much better when the players are actively trying to outsmart each other, and the one who wins is the one who is more creative, not the one who rolls a higher number. I also wouldn't want to see kran prevented from fighting other races. Just because you can't scratch or bruise a kran doesn't mean you can't beat them. If punching a kran would break or damage your hand... don't punch one! Kran may be hard, but they aren't flexible or agile like some races. Use speed, gravity and physics against them. Trip them, climb them, joint lock them... the possibilities are limited by the creativity of the opponent, not by the settings.